Isolation systems are well known in the art, which include passive systems, semi-active systems, and active systems. Passive systems utilize passive isolating devices such as mounts and shock absorbers to isolate a suspended member from shock and vibration inputs. Although these systems provide adequate isolation, they can only dissipate energy from the system. Semi-active systems have been developed which vary the parameters of the isolation system in order to provide better isolation. One such semi-active system incorporating controllable dampers for providing a controllable damper force is described in the commonly assigned U.S. Pat. No. 3,807,678 to Karnopp et al. entitled "System for Controlling the Transmission of Energy Between Spaced Members" which is hereby incorporated by reference herein.
The Karnopp et al. '678 patent describes a passive spring 24 arranged in parallel spring relationship to an active viscous damper 26. The damping characteristics of the damper are varied as a function of feedback signals such as position, velocity, and acceleration. The damping is oscillated in real time between an off state (low damping) and an on state (high damping) responsive to these feedback signals. This on/off damping is accomplished via a control algorithm such as skyhook control, as will be understood by those skilled in the art. However, to accomplish ideal control of the vehicle via skyhook control, assisting as well as resistive forces need to be generated independently of the relative velocity.
A semi-active damper is only capable of generating a resistive damping force, i.e., a dissipative force which is opposed to the direction of travel. For optimum isolation, under certain conditions it is necessary to develop forces which are in the same direction as the direction of travel, i.e., assisting forces. Therefore, when the controller indicates that the desired force for optimum control is assisting, i.e., in the same direction as the damper motion, the response in a semi-active damper is to set the damper to a very low or zero damping (off state) setting. Again, this is because a damper can only provide dissipative forces so the best that the semi-active damper can do is to provide a low magnitude resistive force. As a result, because semi-active dampers are only capable of providing dissipative forces, they can never achieve optimum isolation.
Because of this perceived inadequacy, active systems have developed whereby, active pumps or other sources of active forces have been added to the system to provide the assisting forces dictated by the controller to provide optimum isolation. U.S. Pat. No. 4,625,993 to Williams et al. describes one such active system, whereby connection to a pump provides power to control the system. However, these systems have the disadvantage that they consume large amounts of energy.
To reduce the power requirements of active systems, U.S. Pat. No. 5,098,119 to Williams et al. and U.S. Pat. No. 5,082,308 to Jones describe systems which provide charging an accumulator and later releasing that energy. Although, active systems can provide improved isolation, they are extremely complex and require motors, pumps and the like, which add to the cost and detract from the durability of the system.
U.S. Pat. No. 4,730,816 to Eckert provides an apparatus for controlling spring stiffness of a shock absorber assembly by actuating valve 17 in response to controller 12. The system utilizes a rotary-type valve 17 to direct fluid flow to and from an accumulator 15. Although this system can vary the stiffness of the shock absorber, it cannot provide assisting forces.
Other systems are known which store energy for release at a later time such as the elastomer energy recovery system described in U.S. Pat. No. 4,479,356 to Gill wherein energy is stored via twisting of an elastomeric tube. U.S. Pat. No. 4,359,867 to Swanson describes storing fluid pressure from a vehicle seat suspension such that said pressure can be used as a hydraulic assist to operate the vehicle's clutch upon initial startup. U.S. Pat. No. 4,295,538 to Lewis which describes a system which takes energy from excursions of a vehicle strut to drive a hydraulic motor 48 which in turn drives generator 54 to generate electricity to power electrical devices used in a vehicle. Finally, U.S. Pat. Nos. 5,074,192 to Gheorghita, 5,046,309 to Yoshino, and 4,204,405 to Basham describe regenerative systems which store energy during one portion of a cycle for release in subsequent cycles.